General Description of Hydrodynamic Focusing
The invention discloses a method for the production of multi-component coaxial filaments using a hydrodynamically focused compound liquid jet. A compound liquid jet is defined as a jet propagating with an annular distribution of one or more inner flows surrounded by an outer sheath flow. In hydrodynamic focusing, an annular distribution of a core liquid and a sheath liquid is forced through a channel or nozzle, with the core liquid being stretched into a thin filament as the liquids accelerate through the constriction. The width of the core filament is a function of the ratio of the core and sheath flow rates. In hydrodynamic focusing, the diameter of the core liquid is proportional to the fractional volume occupied by the core liquid. As the ratio of the core liquid flow rate to the sheath liquid flow rate is decreased, the volume occupied by the core liquid decreases, and thus reduces the diameter of the core liquid filament. In hydrodynamic focusing applications, the core liquid is stretched into a filament with a width as small as 1 micron. The core liquid can be a single liquid or an annular distribution of several co-propagating liquids.
The feasibility of a traditional single-component liquid jet printing method depends largely on the ability of the process and apparatus to deliver ink at a useful rate. The maximum practical deposition rate is determined by the maximum velocity and acceleration of the state of the art motion control system. Stable delivery of an ink from a liquid jet apparatus becomes difficult for nozzle diameters less than approximately 50 microns. Indeed, clogging of small diameter nozzles by trapped bubbles or solid particles can be problematic for jetting systems, rendering such nozzles impractical for prolonged use. The invention circumvents the problem of clogging by using a multi-component approach, wherein one or more inner liquids are propelled by an outer sheath flow through a larger diameter external nozzle. The relatively large diameter of the exit nozzle greatly reduces the occurrence of clogging, and helps to stabilize the deposition process. The inner flow is forced through an inner nozzle in a dripping mode, at flow rates on the order of milliliters per hour, while the sheath liquid is transported through an exit nozzle at flow rates of tens of milliliters per hour.
Coaxial Jetting
A coaxial jet is formed when the inner filament and outer sheath liquid are jetted from an output orifice to form an annular liquid jet. In 1969 (U.S. Pat. No. 3,416,730), Perry disclosed an apparatus for the production of multiple liquid jets, initially in contact, but without substantial mixing. Hertz (U.S. Pat. No. 4,196,437) discloses a method and apparatus for forming a compound liquid jet that breaks into droplets in an inkjet printing application. The present invention produces a coaxial jet consisting of two or more annular liquid streams with limited actual contact time, minimizing diffusivity and producing a stable jet with a breakup distance of at least five centimeters.